1. Field of the Invention
This invention is generally concerned with analytical chemistry and, in particular, with the detection of starch in colloidal solution.
2. Description of the Related Art
Electrochemical reactions and techniques are widely used to detect chemical substances and to generate reagents for chemical detection and analyses. These approaches typically involve electrochemical oxidation of a precursor compound to generate a reagent capable of detecting a target analyte, and electrochemical cells with separate compartments have often been employed to generate a reagent from a precursor. In the amperiometric Karl-Fisher titration, for example, which is used to detect residual water in organic solvents, for example, an excess of iodide ion added to the test solution is electrochemically oxidized to iodine which, can react with other test solution components such that water is quantitatively consumed during the process (i.e. one mole of I2 is consumed for each mole of water). The endpoint of the titration is the detection of excess iodine in the specimen, indicating that all of the water has been consumed. In order to avoid any back reduction of iodine, a separate test solution compartment is used for the cathode in this approach.
An excess of precursor is generally added to the solution used for reagent generation in these types of cells, which would be problematic in other applications. It is likely, for example, that a sensor based on the amperiometric Karl-Fisher titration could be developed for detecting the total water content in lubricating oils during service in engines, which is of significant interest to the transportation industry. However, addition of excess reagent precursor to the lubricant, which might degrade the lubricant properties, is undesirable. Furthermore, the presence of excess reagent is usually not a practical alternative since many types of reagents tend to undergo chemical decomposition during storage.
An example of a chemical analysis involving an unstable reagent is detection of starch by reaction with iodine. Starch is the major carbohydrate in plant tubers and seed endosperm, such as corn, wheat, potato, tapioca and rice, so that detection of starch is important to the food industry. Starch is an insoluble complex carbohydrate, a polymeric material having the chemical formula (C6H10O5)n and consisting of various portions of two glucose polymers, amylase and amylopectin. In water, starch usually forms colloidal solutions that turn from a translucent white color to deep blue in the presence of atomic iodine, which adsorbs on the suspended starch colloidal particles to form a starch-iodine complex. The resulting blue-colored starch-iodine complex absorbs light in the wavelength range from about 400 to 800 nm, with a peak (maximum) absorbance for this complex at about 580 nm. In addition, the light absorbance of the complex is proportional to the concentration of starch in solution and the reaction is an extremely specific marker for starch (and potentially for starch analogs or derivatives). Once the available starch is completely complexed with iodine (starch-I2), any further addition of iodine (I2), which is insoluble aqueous solution, usually results in the formation triiodide ion (I3−) which typically imparts a brown color to an aqueous test solution.
A standard method for determining the starch concentration in a colloidal solution is to add an excess of iodine (as a reagent solution) to the colloidal solution and then measure the light absorbance of the resulting test solution at 580 nm. This spectroscopic approach has the disadvantage that the iodine reagent solution is unstable under ambient conditions and must be periodically replaced. This generates an appreciable waste stream, which is exacerbated by the relatively large volume of the iodine reagent needed for the analysis.
For some applications, delivery of the iodine reagent solution can also be problematic, especially for automated on-line analysis. There is, for example, a need for on-line detection of starch in rinse water from sliced potatoes used to manufacture potato chips. A sharp slicing blade is required to provide potato slices of uniform thickness and high quality, but the lifetime of the blade varies. In addition to degrading product quality, a dull slicing blade tends to increase damage to potato cells, which releases starch. An on-line method for monitoring starch in rinse water from the potato slicing operation would enable slicing blades to be changed as needed so as to optimize potato chip quality and blade usage. Early detection of blade degradation would enable blades to be changed at a convenient time to minimize process disruption, saving labor and material costs. Another potential application for on-line starch monitoring is to track the hydrolysis of starch into cyclodextrins. This conversion process, often initiated by readily available enzymes, has application in the food, cosmetic, pharmaceutical, agricultural, environmental, and chemical industries. Monitoring the conversion of starch to cyclodextrins can insure the reaction is complete under various processing conditions and with minimum enzyme consumption. A further application for controlled initiation and monitoring starch-iodide complex is to monitor phosphites in groundwater
Therefore, a method for local injection of a detection reagent provided on demand by a stable reagent precursor reservoir would be advantageous in many analyte detection or analysis applications, particularly where reagents undergo chemical decomposition during storage and where waste stream management or on-line implementation are desirable.